Process for isolating pure diketene with recovery of materials of value

Abstract

The invention relates to a process for isolating pure diketene with recovery of materials of value by continuous, two-stage distillation of crude diketene at reduced pressure and with continuous reaction of the resulting distillation residue with acetic acid, pure diketene and acetic anhydride being separated off by means of the two-stage distillation. In the stabilization of the distillation residue with acetic acid, reutilizable acetic acid, which contains acetone with or without acetic anhydride, is recovered.

What is claimed is: 
     
       1. A process for isolating pure diketene with recovery of materials of value by continuous, two-stage distillation of crude diketene at reduced pressure and with continuous reaction of the resulting distillation residue with acetic acid, which comprises a) in the first distillation stage, feeding the crude diketene to a thin-film evaporator, evaporating it at reduced pressure and feeding the vapor phase to a distillation column, from the top of which pure diketene is drawn off, and combining the column bottoms, which contain a mixture of diketene and acetic anhydride, with the bottom product of the thin-film evaporator,   b) further adding, to the mixture of the bottom products from the thin-film evaporator and the distillation column of the first distillation stage condensate, which is obtained in the production of ketene or the further processing thereof or pure acetic anhydride, which is obtained as bottom product in the distillation in the second distillation stage and recirculated,   c) in the second distillation stage, feeding the combined bottom products to an additional thin-film evaporator, evaporating it at reduced pressure and transferring the vapor phase to a distillation column, from the top of which a mixture containing diketene and acetic anhydride is drawn off and fed back to the distillation column of the first distillation stage, while the bottom product in the form of pure acetic anhydride is transferred out or recirculated, and   d) admixing the bottom product from the thin-film evaporator of the second distillation stage with acetic acid, feeding it to a reactor and, after the reaction has taken place, separating the reaction product in a thin-film evaporator to obtain reutilizable acetic acid, which contains acetone with or without acetic anhydride, as vapor, while an inert residue in liquid form is drawn off as bottom product.   
     
     
       2. The process as claimed in claim 1, wherein the thin-film evaporators and the columns of the two distillation stages are operated at a pressure of from 50 to 100 mbar and at a temperature of from 60° to 85° C. 
     
     
       3. The process as claimed in claim 1 or 2, wherein the thin-film evaporation in the second distillation stage is carried out in such a way that the bottom product has an acetic anhydride content of at least 15% by weight and the bottom product in this form is immediately and continuously stabilized with acetic acid. 
     
     
       4. The process as claimed in claim 1 wherein the reaction with acetic acid is carried out at standard pressure and at a temperature of from 100° to 150° C. 
     
     
       5. The process as claimed in claim 1 wherein acetic acid having a concentration of from 50 to 99.5% by weight is used, the dilution of the acetic acid being calculated such that no water is present in the stabilized residue. 
     
     
       6. The process as claimed in claim 1 wherein the reaction with acetic acid is carried out using from 0.3 to 2.0 parts by weight of acetic acid per 1 part by weight of residue and residence times are from 0.2 to 8.0 hours. 
     
     
       7. The process as claimed in claim 1 wherein for separating the product of the reaction with acetic acid, the thin-film evaporation is carried out at a pressure of from 400 to 500 mbar and at a temperature of from 100° to 180° C.